1. Field of the Invention
This invention relates to a green sand composition for use in casting metals. Particularly, this invention relates to a green sand composition utilizing surface activating characteristics of a pulp waste liquor. More particularly, this invention relates to a green sand composition capable of forming a green sand mold of a high-density, which utilizes the surface characteristics which are obtained by controlling the ratio of saccharide derivatives of hydrolyzed hemi-cellulose and lignins contained in a pulp waste liquor from a digester.
2. Description of the Prior Art
High pressure molding methods have recently been developed as methods which meet the need for increasing the speed of manufacture of castings and improving the dimension precision in castings, and green sand composition suitable for such mass-production methods are now required. Green sand is a casting mold sand comprising as main ingredients siliceous sand, bentonite and water. In this green sand composition, the water content is about 3 to about 4% by weight based on the sand. In general, the molding pressure is about 7 to about 40 kg/cm.sup.2 and the mold is required to have a hardness of at least 90. In the high pressure molding method, since the water content of the green sand composition is lower than that of conventional green sand composition used for the customary molding method, the surfaces of sand particles are crumbly and the sand particles show insufficient stability and adhesion. Accordingly, defects such as buckles, scabs, sand marks and cuts are readily formed on castings and yields of castings are reduced.
Further, since in the high pressure molding method the recycle speed of molding sand is enhanced and the frequency of contact with molten metals is increased, in case of conventional Ca-type bentonite, ignition aging (inactivation due to heat) is accelerated and defective castings are readily formed by sand sintering or sand fusion. Accordingly, Na-type bentonite having a higher fire resistance than that of Ca-type bentonite is now being used instead of Ca-type bentonite. However, since the hydration speed of Na-type bentonite is lower than that of Ca-type bentonite, it is dispersed in the molding sand in the heterogeneous suspended state and it takes a very long time to obtain a uniform green strength (adhesion strength) of the green sand mold by sufficient permeation of water and sufficient wetting with water. Accordingly, use of Na-type bentonite is not suitable for working the high pressure molding method conducted at high speed. Further, Na-type bentonite is inferior to Ca-type bentonite with respect to the disintegrating property of the molding sand after pouring, and in case of Na-type bentonite, solid agglomerates of fine sand and bentonite are readily formed and the subsequent sand treatment becomes difficult. In order to alleviate these defects of Na-type bentonite, for example, about 25 to about 50% by weight of Ca-type bentonite is incorporated into Na-type bentonite, through the fire resistance inherent of Na-type bentonite is sacrificed to some extent. However, no satisfactory synergistic effect can be obtained in this mixture. As generally known in the art, Na-type bentonite indicates bentonite in which the Na-ion exchange capacity (C.E.C.) is greater than the Ca-ion exchange capacity or in which the amount of Na.sub.2 O determined by chemical analysis is greater than that of CaO. On the other hand, Ca-type bentonite indicates bentonite having greater Ca-ion exchange capacity or greater CaO.
In order to obtain a high and uniform mold hardness, it is necessary to improve the flowability and packing property of the green molding sand. For this purpose, there was once adopted a method in which bubbles are formed in the sand by using a surface active agent to render the green sand composition flowable with use of a reduced amount of water and the resulting flowable composition is packed. In this method, however, the bonding power of bentonite is weakened in the recycle system and the mold strength is reduced, resulting in production of defective articles. Therefore, at the present, this method is not practically worked in the art.
The mold to be used in the high pressure molding method and at the high speed is required to have a high hardness. However, in a mold having a high hardness, the part falling in contact with a molten metal is readily destroyed by stress caused by thermal expansion during the casting operation. In order to prevent this defect, an auxiliary binder such as starch, alkali polyuronate, sodium carboxymethyl cellulose, sodium polyglycolate or the like has heretofore been incorporated in the green sand in the powdery state. In this case, however, the degree of wetting with water is greatly changed depending on the mulling (tempering) time, and therefore, it is difficult to attain a uniform scab-preventing effect. By the term "auxiliary binder" is meant a binder to be added to improve the properties of the green sand, which has an activity of assisting the main binder (bentonite).
U.S. Pat. No. 3,086,874 discloses that additive material selected from the group consisting of modified starches and starch derivatives is used in order to prevent rupture or separation of the surface layer or crust of a mold when it is heated by a molten metal being poured into the mold. U.S. Pat. No. 3,086,875 discloses a mixture of a material selected from the group consisting of a starches, modified starches and starch derivatives and a cellulosic material which has been specially treated.
Still further, as additives for assisting the fire resistance of siliceous sand, preventing sand sintering and improving surface conditions of castings, there have heretofore been used carbonaceous additives, for example, coal powders (such as bituminous coal, brown coal, (lignite), peat and sea coal powders), bituminous powders (such as tar pitch powder and asphaltene powder), graphite powder, electrode carbon dust, rosion and synthetic resin powders (inclusive of waste synthetic resin powders such as burrs and liners), having a size of 150.+-.50 mesh (Tyler). Most of these additives do not adhere chemically or physically to the sand surface but they are randomly dispersed among sand particles. Accordingly, these additives are accumulated in excessive amounts in the green sand composition to cause undesirable increase of the carbon content. Further, when such powdery additive is incorporated into the green sand composition, it is mixed with bentonite and hydrated and hence, "spring back" is manifested in the composition to reduce the flowability and air permeability of the green sand. U.S. Pat. No. 3,023,113 discloses a carbonaceous material composed of a mixture of lignite and an aqueous emulsion of asphaltic pitch, and a carbonaceous substance selected from the group consisting of wood flour, coke breeze and mixture thereof and the like.